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| United States Patent |
5,149,349
|
|
Berkey
, et al.
|
September 22, 1992
|
Method of making polarization retaining fiber with an elliptical core,
with collapsed apertures
Abstract
Disclosed is a method of making a polarization retaining single-mode
optical fiber. There is initially formed a draw blank having diametrically
opposed longitudinal apertures in the cladding glass parallel to the core
glass region. The draw blank is drawn into a fiber under such conditions
that the apertures close as the fiber is being drawn. The flow of
surrounding glass, including the core glass region, toward the collapsing
apertures, causes the core to assume an elliptical shape. The apertures
are of such cross-sectional area and spacing from the core that the core
develops the desired aspect ratio.
| Inventors:
|
Berkey; George E. (Pine City, NY);
Hawk; Robert M. (Bath, NY);
Tarcza; Steven H. (Painted Post, NY)
|
| Assignee:
|
Corning Incorporated (Corning, NY)
|
| Appl. No.:
|
728276 |
| Filed:
|
July 11, 1991 |
| Current U.S. Class: |
65/403; 65/410 |
| Intern'l Class: |
C03B 037/023 |
| Field of Search: |
65/3.11,3.12,31,3.2,18.2
385/146,11,123
156/663
|
References Cited
U.S. Patent Documents
| 4395270 | Jul., 1983 | Blankenship | 65/3.
|
| 4493530 | Jan., 1985 | Kajioka | 350/96.
|
| 4561871 | Dec., 1985 | Berkey | 65/3.
|
| 4709986 | Dec., 1987 | Hicks | 65/3.
|
| 4859223 | Aug., 1988 | Kajioka | 65/3.
|
| 4978377 | Dec., 1990 | Brehm | 65/3.
|
| Foreign Patent Documents |
| 2930704 | Feb., 1981 | DE | 65/3.
|
| 0145632 | Aug., 1983 | JP | 65/3.
|
Other References
Kingery et al., Introduction to ceramics, 1976, pp. 469-470, 501, John
Wiley & Sons, ed.
|
Primary Examiner: Jones; W. Gary
Assistant Examiner: Hoffmann; John
Attorney, Agent or Firm: Simmons, Jr.; William J.
Claims
I claim:
1. A method of making a polarization retaining single-mode optical fiber
comprising drawing an optical fiber from a cylindrically-shaped draw blank
having a cylindrically-shaped glass core surrounded by cladding glass and
having apertures consisting of a single pair of cylindrical apertures that
are parallel to and diametrically opposed with respect to said core, said
fiber being drawn at such a rate that said apertures close, thus causing
the core of said fiber to have an elliptical cross-section.
2. A method according to claim 1 wherein the cross-sectional area of each
of said apertures is such that the cross-section of said fiber is
circular.
3. A method according to claim 1 wherein the cross-sectional area of each
of said apertures is such that the outer surface of said fiber has two
opposed sides that are flatter than the remainder of said fiber.
4. A method according to claim 1 wherein said apertures are evacuated
during the step of drawing.
5. A method according to claim 1 wherein said draw blank is formed by
forming longitudinal grooves on diametrically opposed sides of a
cylindrically-shaped core preform in which said glass core is surrounded
by said cladding glass,
inserting said core preform into a glass tube to form an assembly having
enclosed grooves on diametrically opposed sides of said core,
shrinking said tube onto said core preform,
fusing said core preform to said tube to thereby form a consolidated
assembly containing said apertures, and
removing a portion of said core preform adjacent said grooves and a portion
of said tube adjacent said grooves to enlarge the cross-sectional area of
said enclosed grooves and form said apertures.
6. A method according to claim 5 wherein the step of enlarging comprises
flowing an etchant through said apertures.
7. A method of making a polarization retaining single-mode optical fiber
comprising the steps of
forming longitudinal grooves on diametrically opposed sides of a
cylindrically-shaped core preform having a glass core surrounded by
cladding glass,
inserting said core preform into a glass tube,
shrinking said tube onto said core preform,
fusing the interface between said core preform and said tube, thereby
forming a consolidated assembly having longitudinal apertures that are
parallel to said core to form a draw blank, and
drawing an optical fiber from the resultant draw blank at such a rate that
said apertures close, thus causing the core of said optical fiber to have
an elliptical cross-section.
8. A method according to claim 7 wherein said tube has outer and inner
surfaces and wherein, during the steps of shrinking and fusing, said
method comprises applying a pressure differential across said tube,
whereby the pressure on the outer tube surface exceeds that on the inner
tube surface, the pressure differential across said tube being sufficient
to collapse said tube onto said core preform, but being insufficient to
cause glass to flow into and eliminate said grooves.
9. A method according to claim 7 wherein the steps of shrinking and fusing
comprise depositing glass particles on the outer surface of said tube, and
heating and consolidating said particles, thereby exerting on said tube a
radially inwardly directed force that causes said tube to shrink onto said
core preform, the step of heating also fusing said tube to said rod.
10. A method according to claim 9 further comprising the step of flowing an
etchant through said apertures to increase the cross-sectional areas
thereof.
11. A method according to claim 10 wherein said apertures are evacuated
during the step of drawing.
12. A method according to claim 9 wherein said apertures are evacuated
during the step of drawing.
13. A method of making a polarization retaining single-mode optical fiber
comprising the steps of
forming longitudinal grooves on diametrically opposed sides of a
cylindrically-shaped core preform having a glass core surrounded by
cladding glass,
depositing glass particles on the outer surface of a glass tube,
inserting said core preform into said glass tube to form an assembly,
heating said assembly to consolidate said particles, thereby exerting on
said tube a radially inwardly directed force that causes said heated tube
to shrink onto and fuse to said core preform, thereby forming a
consolidated assembly having longitudinal apertures that are parallel to
said core to form a draw blank, and
drawing an optical fiber from the draw blank at such a rate that said
apertures close, thus causing the core of said optical fiber to have an
elliptical cross-section.
14. A method according to claim 13 wherein said apertures are evacuated
during the step of drawing.
15. A method according to claim 13 further comprising the step of flowing
an etchant through said apertures to increase the cross-sectional areas
thereof.
16. A method according to claim 15 wherein said apertures are evacuated
during the step of drawing.
17. A method of making a polarization retaining single-mode optical fiber
comprising drawing an optical fiber from a cylindrically-shaped draw blank
having a cylindrically-shaped glass core surrounded by cladding glass and
having apertures that are parallel to and diametrically opposed with
respect to said core, said fiber being drawn at such a rate that said
apertures close, thus causing the core of said fiber to have an elliptical
cross-section, said draw blank being formed by the following steps:
forming longitudinal grooves on diametrically opposed sides of a
cylindrically-shaped core preform in which said glass core is surrounded
by said cladding glass,
inserting said core preform into a glass tube to form an assembly having
enclosed grooves on diametrically opposed sides of said core, said tube
having outer and inner surfaces,
shrinking said tube onto said core preform to form said apertures,
fusing said core preform to said tube, thereby forming a consolidated
assembly containing said apertures,
applying a pressure differential across said tube during the steps of
shrinking and fusing, whereby the pressure on the outer tube surface
exceeds that on the inner tube surface, the pressure differential across
said tube being sufficient to collapse said tube onto said core preform,
but being insufficient to cause glass to flow into and eliminate said
grooves, the step of applying a pressure differential across said tube
comprising depositing glass particles on the outer surface of said tube,
heating the resultant assembly to consolidate said particles, the process
of consolidating said particles exerting a force radially inwardly on said
tube, thereby causing said tube to shrink onto said core preform, the step
of heating also fusing said tube to said core preform.
Description
BACKGROUND OF THE INVENTION
This invention relates to the fabrication of polarization retaining
single-mode (PRSM) optical fibers and more particularly to the fabrication
of preforms from which fibers having elliptically-shaped cores can be
drawn.
In many applications of single-mode optical fibers, e.g. gyroscopes,
sensors and the like, it is important that the propagating optical signal
retain the polarization characteristics of the input light in the presence
of external depolarizing perturbations. This requires the waveguide to
have an azimuthal asymmetry of the refractive index profile.
One of the first techniques employed for improving the polarization
performance of single-mode fibers was to distort the symmetry of the core.
One such optical fiber is disclosed in the publication by V. Ramaswamy et
al., "Influence of Noncircular Core on the Polarization Performance of
Single Mode Fibers", Electronics letters, Vol. 14, No. 5, pp. 143-144,
1978. That publication reports that measurements made on such fibers
indicated that the noncircular geometry and the associated stress-induced
birefringence alone were not sufficient to maintain polarization in
single-mode fibers.
Fiber cores having a relatively high aspect ratio are required to obtain
adequate polarization retaining properties. A high core/clad .DELTA. also
improves these properties. Techniques which have been developed for
improving core ellipticity are subject to various disadvantages. Some
techniques are not commercially acceptable because of their complexity.
Double crucible techniques result in fibers having relatively high
attenuation. Some techniques employ very soft glasses for certain fiber
portions, and those soft glasses are detrimental to the propagation of
light at long wavelengths where the core glass would normally experience
extremely low attenuation. Soft glasses can also complicate the fusion
splicing of fibers, since the soft glass flows too readily when the fibers
are heated during the splicing operation.
SUMMARY OF THE INVENTION
It is therefore an object of the invention to provide a method of making
PRSM optical fibers which overcomes the disadvantages of the prior art. A
further object is to provide a PRSM fiber producing method which is
relatively simple to practice and which can employ glasses which do not
detrimentally affect light attenuation. Yet another object is to provide a
method that can produce PRSM fibers, the outer surfaces of which are
either round or flattened, depending upon the requirements of the
particular product.
In accordance with the present method, a polarization maintaining
single-mode optical fiber is formed by drawing a fiber from a draw blank
having a glass core surrounded by cladding glass containing apertures that
are diametrically opposed with respect to the core. The fiber is drawn at
such a rate and temperature that the apertures close and the core becomes
elliptical. Draw rate can be increased by evacuating the apertures during
drawing. Core ellipticity can be controlled by controlling the
cross-sectional area of the apertures as well as the spacing between the
core and the apertures in the draw blank. Furthermore, the drawn fiber can
be formed with a circular cross-section or one having opposed flattened
sides, depending on the size of the apertures and their spacing from the
core.
In a preferred method of making the draw blank, longitudinal grooves are
formed on diametrically opposed sides of a cylindrically-shaped core
preform in which the glass core is surrounded by the cladding glass. The
core preform is inserted into a glass tube, and the tube is collapsed and
fused to the grooved core preform to form an assembly having longitudinal
apertures on opposite sides of the core.
During the step of shrinking the tube onto the grooved core preform, it is
advantageous to apply a differential pressure across the tube, whereby the
pressure on the outer surface exceeds that on the inner surface. This can
be accomplished by depositing glass particles on the outer surface of the
tube, heating the resultant assembly to consolidate the particles, the
process of consolidating the particles exerting a force radially inwardly
on the tube, thereby causing the tube to shrink onto the core preform, the
step of heating also fusing the tube to the core preform.
The cross-sectional size of the apertures can be precisely controlled by
initially forming the apertures smaller than desired and thereafter
enlarging the cross-sectional area of the apertures while checking their
dimensions. Aperture enlargement can be accomplished by flowing an etchant
therethrough.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a cross-sectional view of a preform from which an elliptical core
PRSM fiber can be drawn.
FIG. 2 is a schematic diagram illustrating the drawing of a PRSM fiber from
the preform of FIG. 1.
FIG. 3 is a cross-sectional view of a PRSM fiber produced by the present
method.
FIG. 4 is a cross-sectional view of a grooved core preform.
FIG. 5 is a schematic diagram illustrating the drawing of a rod from the
grooved core preform of FIG. 4.
FIG. 6 illustrates the application of a coatings of glass particles to a
glass tube.
FIG. 7 is a cross-sectional view illustrating the consolidation and fusion
of a first assembly.
FIG. 8 is a cross-sectional view taken along lines 8--8 of FIG. 7.
FIG. 9 is a cross-sectional view of a preform resulting from the
consolidation/fusion step illustrated in FIG. 7.
FIG. 10 is a cross-sectional view illustrating the circulation of etchant
through the apertures of preform.
FIG. 11 is a cross-sectional view illustrating the application of first and
second coatings of porous glass to a mandrel.
DESCRIPTION OF THE PREFERRED EMBODIMENT
It is to be noted that the drawings are illustrative and symbolic of the
invention, and there is no intention to indicate scale or relative
proportions of the elements shown therein.
Draw blank 10 of FIG. 1, from which a PRSM fiber can be drawn, has core and
cladding regions 11 and 12, respectively. The core and cladding regions
may be formed of conventional materials employed in the formation of
optical waveguides. The salient characteristics of these materials are
that the refractive index of the core material must be greater than that
of the cladding material and that both materials must exhibit low losses
at the wavelength at which the waveguide is intended to be operated. By
way of example only, core region 10 may consist of pure silica or silica
containing one or more dopants which increase the refractive index
thereof. Region 11 may consist of pure silica, silica containing a lesser
amount of dopant than core region 11, or silica containing one or more
dopants, at least one of which is an oxide of an element such as boron or
fluorine which lowers the refractive index of silica. Although silica is a
preferred base glass because it exhibits low loss at useful wavelengths,
base glass materials other than silica may be employed.
Apertures 13 extend longitudinally through blank 10 parallel to core region
11. While apertures 13 are illustrated as being circular in cross-section,
the cross-sectional configuration could be crescent shaped, D-shaped, or
the like. Any shape that results in the desired cross-sectional elongation
of the core during fiber draw is considered to be suitable.
Referring to FIG. 2, draw blank 10 is placed in a conventional draw furnace
where tractors 17 pull fiber 15 from the bottom portion of blank 10 which
is heated to draw temperature by heating elements 16. The tendency for
apertures 13 to close is a function of draw rate and glass viscosity. The
viscosity of the draw blank root from which the fiber is drawn depends
upon furnace temperature and glass composition. If the viscosity of the
heated portion of the blank is sufficiently low and the draw rate is
sufficiently low, apertures 13 will naturally close during the draw
process. Since the apertures more readily close if they are evacuated,
draw speed can be increased by affixing a vacuum attachment 18 to the
upper end of the blank. Vacuum also reduces the possibility of core
contamination by hydroxyl groups during the high temperature fiber draw
step.
As apertures 13 close, they are replaced by the surrounding glass. When
glass at smaller radii than the apertures flows radially outwardly into
the apertures, core region 11 becomes elongated in cross-section. The
resultant PRSM fiber 15, the cross-section of which includes cladding 22
and oblong core 21 is shown in FIG. 3. The ellipticity or aspect ratio of
the elliptical core is the ratio of its major dimension to its minor minor
dimension in a plane perpendicular to the fiber axis. Cores of varying
degrees of ellipticity can be made depending on the size of apertures 13
and the spacing between those apertures and the core. Assume that in draw
blank 10 of FIG. 1, apertures 13 have an area A and a spacing S between
each aperture and core 11. Assume further that these parameters result in
a core ellipticity of X:1. If S is increased, and all other parameters
remain the same, fiber core ellipticity will be less than X:1. If A is
increased, and all other parameters remain the same, core ellipticity will
be greater than X:1. Suitable values of ellipticity can be obtained with
values of A and S that are sufficiently small that the drawn fiber retains
the circular shape of the preform. Circular fibers are preferred for
certain applications.
Ellipticity can also be X:1 at some spacing slightly greater than S and
some area slightly greater than A. However, at some value of S, and at a
corresponding value of A needed to achieve a desired ellipticity, the
outer surface of the preform will begin to collapse inwardly to such an
extent that the fiber will be out-of-round. This feature may have utility
for certain applications; for example, the outer surface of the fiber can
be used to orient the direction of the major axis of the core.
Apertures 13 must be parallel to the core and uniform in diameter and
radius throughout the longitudinal axis of draw blank 10 if fiber 15 is to
have uniform properties throughout its length. Any conventional technique
that meets these requirements can be used for forming the apertures. UK
Patent Application GB 2,192,289 teaches two techniques for forming
longitudinal holes in a preform on opposite sides of the core:
(1) The holes can be drilled with a diamond drill.
(2) A core preform having opposed flattened sides is placed in the center
of a glass tube, and two glass rods are placed on opposite sides of of the
core preform, leaving two opposed unfilled regions between the core
preform and the tube. The resultant assembly is drawn to reduce the
diameter thereof and to cause the glass members to fuse together to form
an article that has a solid cross-section except for two opposed axe-head
shaped holes that correspond to the unfilled regions.
A preferred method of making draw blank 10 is illustrated in FIGS. 4-10.
Referring to FIG. 4, there is initially provided a glass single-mode core
preform 30, i.e. a preform in which the ratio of the diameter of core 31
to the diameter of cladding 32 is greater than that which is required to
draw a single-mode fiber from the preform. In order to form a single-mode
optical fiber from such a core preform, it is conventionally overclad with
additional cladding glass to provide the desired ratio of core diameter to
cladding diameter. Preform 30 can be made by any known technique such as
modified chemical vapor deposition (MCVD), vapor axial deposition (VAD)
and outside vapor deposition (OVD). The refractive index profile of the
core can be step-type, graded or the like.
Longitudinally-extending grooves 34 are formed in cladding 32 on opposite
sides of core 31 by means such as grinding, sawing or the like. After the
grinding operation, the grooved preform is preferably etched and rinsed to
remove particulate matter. If the diameter of the grooved core preform is
too large for subsequent processing, it is inserted into the apparatus of
FIG. 5, a conventional draw furnace where its tip is heated by means 38.
One end of silica rod 39 is fused to the lower end of the preform, and the
other end of the rod is engaged by motor-driven tractor 40. A grooved rod
41 having a core 31', cladding 32' and longitudinal grooves 34' is drawn.
An end 49 of cladding tube 47 is tapered as shown in FIG. 7, and a glass
plug 50 is fused to the tapered end. Referring to FIG. 6, the ends of tube
47 are then mounted in a lathe where it is rotated and translated with
respect to soot deposition means 45. Particles 46 of glass soot are
deposited on tube 47 to build up coating 48. Soot 46 preferably has the
same composition as tube 47.
As shown in FIG. 7, a section 42 of the grooved rod 41 is inserted into the
end of tube 47 opposite tapered end 49 until it contacts the tapered end,
thereby forming assembly 52. End 54 of tube 47 is tapered and is then
fused to handle 55. While assembly 52 is lowered into consolidation
furnace muffle 51, a drying gas flows upwardly through the muffle (arrows
53). The drying gas conventionally comprises a mixture of chlorine and an
inert gas such as helium.
As soot coating 48 consolidates, it exerts a force radially inwardly on
tube 47, thereby forcing that tube inwardly against section 42. A lower
density soot will provide a greater force; however, the soot coating must
be sufficiently dense to prevent cracking. As shown in FIG. 9, the
resultant consolidated assembly 58 comprises core 31' surrounded by
cladding 59. The original cladding region 32' and tube 47 are completely
fused at dashed line 60. Porous glass coating 48 has become completely
consolidated and fused to tube 47 as indicated by dashed line 61. Grooves
34' have become apertures 57 which are parallel to the longitudinal axis
of assembly 58.
After consolidation, the bottom end of assembly 58 is severed to form
endface 63 (FIG. 10). Consolidated assembly 58 can be drawn directly into
a fiber if the cross-sectional areas of apertures 57 are sufficiently
large. If the cross-sectional areas of apertures 57 are too small, they
can be enlarged by flowing a liquid etchant such as HF through them.
Etchant is pumped from reservoir 56 into tube 62 that is affixed to handle
portion 55 of consolidated assembly 58. The etchant flows through
apertures 57, and, as indicated by arrows 67, it flows back into reservoir
56 from which it is recirculated by pump P. Assembly 58 can be
periodically removed from the etching apparatus and checked with a tapered
gauge to ascertain the size of the apertures.
In an alternative aperture etching method, the consolidated preform is
lowered into a consolidation furnace muffle while an etchant gas such as
NF.sub.3, SF.sub.6 or the like flows through the handle, into the top of
tube 47 and through apertures 34', thereby etching and enlarging the
aperture walls. The etchant SF.sub.6 is preferred since it acts more
slowly, thus providing greater control. A preferred furnace for this
process is the scanning consolidation furnace disclosed in U.S. Pat. No.
4,741,748. Such a furnace is capable of providing a sharp hot zone, and
its temperature is readily adjustable. The size of the aperture formed by
the etching process depends on temperature, etchant flow rate and rate at
which the heating coil scans upwardly along the preform.
The resultant draw blank is inserted in a draw furnace, and a vacuum
attachment is connected to handle 55. The lower end of the blank is then
sealed; this can be accomplished by heating the end of the blank and
dropping a gob therefrom. The apertures are then evacuated, and the fiber
is drawn.
The combined thicknesses of tube 47 and soot coating 48 are sufficient
that, when those glass layers are combined with the thickness of preform
cladding layer 32, the resultant optical fiber exhibits the desired
single-mode properties. The thickness of cladding layer 32 is sufficient
to locate apertures 34' the proper distance from core 31'. This distance
depends upon the desired aspect ratio of the resultant fiber core.
Instead of employing soot coating 48 to exert the necessary force to cause
complete fusion of tube 47 to preform 30, a low level vacuum could be
applied to tube 47 while the assembly of core preform 30 and tube 47 is
gradually inserted into a furnace having a narrow hot zone. This can be
accomplished by affixing a vacuum attachment to one end of the assembly,
and sealing the grooves at the opposite end. Alternatively, the ends of
tube 47 could be sealed in a chamber that applies pressure to the outside
walls of the tube. As tube 47 is heated, the pressure collapses it onto
preform 30.
The aperture forming technique of FIGS. 4-10 is advantageous in that it
creates accurately sized apertures that are parallel to the longitudinal
axis of the draw blank. Aperture shapes such as square, U-shaped, V-shaped
or the like, can be formed by initially forming the appropriately shaped
groove in core preform 30 of FIG. 4. For example, U-shaped apertures can
be formed by grinding U-shaped grooves in a core preform, inserting the
preform into a tube, and then heating the assembly to collapse the tube
and shrink it onto the preform. If desired, the apertures can be subjected
to a mild etch to smooth the walls thereof, the etching step being
insufficient to enlarge and change the aperture shape to round. A strong
etchant such as NF.sub.3 can change the aperture shape to round.
The following example illustrates the manner in which the method of the
present invention can be employed to produce polarization retaining
single-mode optical fibers. An optical fiber core preform was formed by a
method similar to that disclosed in U.S. Pat. No. 4,486,212 which is
incorporated herein by reference. Referring to FIG. 11, the large diameter
end of an alumina mandrel 87 was inserted into glass tube 88. The outside
diameter of the mandrel tapered from 5.5 mm to 6.5 mm over its 107 cm
length. The ends of mandrel 87 were mounted in a lathe where it was
rotated and translated.
The face of burner 45, which was of the type disclosed in U.S. Pat. No.
4,165,223, was positioned 13.7 cm from mandrel 87. Reactant compounds
emanating from the central burner orifice were oxidized in the flame to
form glass particle stream 46. Auxiliary burners 90 directed flames toward
the ends of the porous glass preform during deposition. The use of
auxiliary burners is taught in U.S. Pat. No. 4,810,276.
The system for delivering the gas-vapor mixture to the burner was similar
to that disclosed in U.S. Pat. No. 4,314,837. Liquid SiCl.sub.4 was
maintained at 79.degree. C. in a first container, and liquid GeCl.sub.4
was maintained at 100.degree. C. in second container, thus producing vapor
at about 20 psi. During the deposition of the preform, vapors were metered
from the first and second containers and were premixed with oxygen before
being supplied to the burner.
The burner traversed a 49 cm section of mandrel in 25 seconds. An acetylene
torch supported on the burner was first employed to deposit carbon
particles on mandrel 87 during one burner pass to facilitate removal of
the porous preform. A porous core preform 93 was then formed by traversing
burner 45 along mandrel 87 many times with respect to burner 45 to cause a
build-up of many layers of soot. During the entire 310 minute run,
SiCl.sub.4 flowed to burner 45 at a rate of 0.9 slpm. During the 300
minute deposition of core region 92, GeCl.sub.4 flowed to the burner in
accordance with the following schedule: (a) 0.75 slpm during the first 150
minutes, (b) a linear ramp from 0.75 to 0.65 slpm during the next 50
minutes, (c) a linear ramp from 0.65 to 0.53 slpm during the next 50
minutes, and (d) a linear ramp from 0.53 to 0.13 slpm during the next 50
minutes. The GeCl.sub.4 was turned off, and only 0.9 slpm SiCl.sub.4
flowed to the burner during the last 10 minutes of the run to form thin
silica coating 91.
The preform was removed from the lathe, and the mandrel was removed through
tube 88, thereby leaving a longitudinal aperture in the porous preform.
Protrusions 89 caused tube 88 to adhere to the preform; that tube remained
at one end of the preform to provide support for subsequent processing.
The preform was then dried and consolidated in accordance with the
teachings of U.S. Pat. No. 4,125,388. A short length of capillary tube was
inserted into the bottom of the porous preform aperture. A drying gas
consisting of 5 volume percent chlorine and 95 volume percent helium was
flowed through tube 88 and into the preform aperture. A helium flushing
gas flowed upwardly through the consolidation furnace muffle. The preform
was gradually lowered into a consolidation furnace muffle, thereby forming
a consolidated preform having a diameter of 52 mm.
The consolidated preform was inserted into the draw apparatus of FIG. 5
where its tip was heated to 1900.degree. C. A vacuum connection was
affixed to its upper end. After the end of the preform was stretched so
that its aperture was either very narrow or completely closed, the
aperture was evacuated. As the lower end of the preform was pulled
downwardly at a rate of about 15 cm/min, and its diameter decreased, the
evacuated aperture collapsed. The diameter of the resultant rod was 6 mm.
A plurality of 90 cm sections were severed from the rod, and one of the
sections was supported in a lathe where it functioned as a mandrel for the
deposition of additional silica cladding soot. This outer cladding was
formed by flowing SiCl.sub.4 vapor to the burner at a rate of 2 slpm for
300 minutes. This overclad process continued until a coating of SiO.sub.2
soot having an outside diameter of 70 mm was deposited to form a composite
preform. The composite preform was consolidated at 1450.degree. while a
mixture of 98.75 volume percent helium and 1.25 volume percent chlorine
flowed upwardly through the muffle. The resultant consolidated core
preform had a diameter of 40 mm and a core diameter of about 6 mm.
A grinding wheel was employed to form longitudinal grooves 34 in cladding
32 on opposite sides of core 31 of the resultant preform 30 (FIG. 4). The
groove dimensions were 0.5 inch (1.27 cm) wide by 0.375 inch (9.5 mm)
deep. Before stretching, the grooved preform etched and rinsed. The
grooved core preform was inserted into the apparatus of FIG. 5 where its
tip was heated to 1900.degree. C. Grooved rod 41, having an outside
diameter of 5 mm, was drawn from preform 30. Rod 41 was severed into 30 cm
sections 42 which were cleaned with HF for 20 minutes and rinsed in
deionized water.
One end of a 100 cm long piece of silica cladding tube 47 having a 5.3 mm
inside diameter and 8 mm outside diameter was tapered at end 49 (FIG. 7),
and silica plug 50 was fused to the tapered end. The ends of the resultant
structure were mounted in a lathe where it was rotated and translated with
respect to flame hydrolysis burner 45 (FIG. 6). Particles 46 of SiO.sub.2
soot entrained in the burner flame were deposited on tube 47 to build up a
1000 gram coating 48 having a length of 70 cm and an outside diameter of
70 mm. The grooved core preform 42 was inserted into the opposite end of
tube 47 until it contacted the tapered end. End 54 of tube 47 was tapered
and was then fused to handle 55. While assembly 52 was rotated at 1 rpm,
it was lowered into consolidation furnace muffle 51 at a rate of 5 mm per
minute. During consolidation, assembly 52 was subjected to a temperature
of 1460.degree. C., and a gas mixture comprising 400 cc chlorine and 20
lpm helium flowed upwardly through the muffle. As soot coating 48
consolidated, it forced tube 57 inwardly against section 42, and the
contacting surfaces of tube 57 and section 42 became fused. An end was
severed from the consolidated preform to form planar endface 63 (FIG. 10).
Tubing 62 was inserted into handle 55, and 50% HF was pumped through
apertures 57 to enlarge them (see FIG. 9). Assembly 58 was periodically
checked with a tapered gauge to ascertain the size of the apertures. The
minimum cross-sectional dimension of apertures 57, which was initially 0.8
mm, was enlarged to 2.5 mm after about 12 hours of etching. The assembly
was then rinsed in deionized water and allowed to dry.
The resultant draw blank was inserted in a draw furnace, and a vacuum
attachment was connected to its handle. With the furnace temperature set
at 2050.degree. C., a polarization retaining single-mode optical fiber was
drawn at a rate of 3 m/sec. The circumference of the fiber was round.
While the fiber was being drawn, two acrylate coatings were applied to it
to form a 172 .mu.m thick composite coating.
The core dimensions along its major and minor axes were 3.5 .mu.m and 0.6
.mu.m, respectively; its aspect ratio was 5.8:1. The outside diameter of
the fiber was 80 .mu.m. The attenuation of the fiber was 8 dB/km at 780
nm. The beat length was 0.9 mm and the h parameter was equal to or less
than 2.times.10.sup.-5.
A similarly formed fiber exhibited a beat length of 2.2 mm at 1550 nm and
an attenuation of 1.3 dB/km at 1500 nm.
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